In-body wireless communications techniques are limited by the ability of living tissue to conduct electromagnetic energy. The generally accepted range of frequencies wherein attenuation by the tissue is acceptable is 40 kHz to less than 30 MHz. This range is also limited to a subset of frequencies where strong interfering signals such as AM radio, WWV, marine radio, etc. are minimal The practice is further limited by the need for both the transponder and monitoring equipment to be physically small. In the case of the transponder, the target application might be for example, monitoring the internal pressure of the bladder. In this case, the transponder will most likely be inserted through the urethra and thus must be small to avoid damaging the urethra during insertion. In the case of the monitoring equipment, the equipment may by necessity be handheld or patent worn to allow mobility. Particularly limiting is the physical size of the transmitter and receiver antennas. With the given spectrum constraints, these antennas must be less than the wave length of the target frequency and are usually of a loop design. Given this restraint, the electromagnetic energy transmitted has the electric field component almost completely suppressed leaving only the magnetic field. The field strength of the resulting inductive field is attenuated at a rate proportional to the inverse of the distance from the source raised to the third power. This in turn limits the useful range to short distances. However, for most applications communication over a distance of 24 to 30 inches is adequate.
Typical parametric monitoring applications include but are not limited to the following:                Pressure in the bladder, veins and arteries, cranial cavity, etc.        Strain induced in mechanical implants such as hip and knee replacements.        Temperature at a specific body location or in a body cavity.        Clinically significant chemicals concentrations such as oxygen, carbon dioxide, glucose, etc.        Flow and transportation rates in vein and arteries, the gastric track, etc.        
The second medical application is the detection of surgical implements inadvertently left behind in a surgical site, a situation that leads to a condition known as gossypiboma. Despite best efforts such as counting implements before and after a surgical procedure, it is not uncommon for implements to be left in the surgical wound. When the “counts” are found to be off, the common practice is to x-ray the patient to locate the missing implement and to this end implements have been designed to have radio-opaque markers. This subjects the patient to longer delays on the operation table and does not always detect the implement. It also fails to address the situation where the “count” is on but there is still an implement in the wound. This case is estimated cause 50 to 70% of the cases.
The current invention is applied to the problem by tagging each implement with a transponder. Before closing the surgical wound, the region is scanned with a handheld embodiment of the invention that simply indicates if a tagged implement is in the field. In order to be successful the system must always detect tagged implements while rejecting signals such as those from eddy currents induced in metal implants or signals from other sources.